1. Field of the Invention
The invention is in the field of adhesives or binders suitable for use under conditions of heat and pressure in binding cellulosic materials and processes for the preparation of molded objects such as wood particle board or other pressed or molded products.
2. Description of the Prior Art
Cellulosic particle board is made by pressing comminuted cellulosic material which can be vegetable fiber such as cornstalks, straw or cellulosic materials derived from trees, i.e., wood pulp, shredded paper or bark. The binders commonly used in producing particle board are thermosetting resins such as urea-formaldehyde resins or other thermosetting resins such as phenol-formaldehyde, resorcinol-formaldehyde and melamine-formaldehyde. The thermosetting resins exemplified by the urea-formaldehyde and phenol-formaldehyde resins are generally used at levels of between 5 and 15% based upon the dry weight of the cellulosic material.
It is known to use organic polyisocyanates as binders in processes for the preparation of wood particle board in which the organic polyisocyanates can be used alone or in combination with formaldehyde. The use of organic polyisocyanates as binders and/or impregnating agents for lignocellulosic materials is disclosed in German Pat. No. 2,109,686, in U.S. Pat. No. 3,870,665 and in U.S. Pat. No. 3,919,017. The limitations of the use of an organic polyisocyanate as a binder for lignocellulosic material are specifically described in U.S. Pat. No. 3,870,665. The known tendency of these resins to stick to metallic molds can be overcome by the incorporation of a catalyst as described which promotes the reaction of NCO groups to form isocyanurate rings. The catalyst need only be incorporated into the mold-release agent which is coated onto the metal parts of the mold used in pressing the lignocellulosic material to overcome sticking to the metallic mold face according to the teaching of this patent.
Combinations of an organic polyisocyanate and formaldehyde are disclosed in U.S. Pat. No. 3,919,017 as binders for cellulosic materials for use in the consolidation of such materials by the use of elevated pressure and temperature conditions. The severe sticking of polyisocyanate adhesives to metal is noted. The proposed solution is to provide a release film or an anti-stick coating on exposed metal parts.
It is also known to utilize the reaction product of an organic polyisocyanate with a polyether polyol as a binder for polyvinyl chloride foam from British Pat. No. 1,303,516. This reference does not teach such a reaction product as a binder for cellulosic particles in a process for the production of particle board or other heat and pressure molded cellulosic articles.